1. Field of the Invention
The present invention relates to a light emitting diode for optical communication, and more particularly, to a light emitting diode having a high response.
2. Description of the Related Art
FIG. 3 is a sectional view showing a conventional light emitting diode used as a light emitting source of a transmission side in an optical communication system. The light emitting diode typically has the following structure.
(1) A light emitting layer 5 is sandwiched between clad layers 4 and 6 each having a refractive index smaller than that of the light emitting layer 5. PA1 (2) The emitted light is extracted from one of the clad layers 4 and 6. PA1 (3) A current injected into the light emitting layer is limited to an internal portion of a circular hole having a diameter of several tens .mu.m.
The diode is actually provided as follows. Reference numeral 1 denotes a substrate of p-type GaAs crystal having a thickness of 100 [.mu.m]; 2, a current blocking layer of n-type GaAs crystal having a thickness of 1.5 [.mu.m]; 3, a circular hole formed in the current blocking layer and having a diameter of 40 [.mu.m] and a depth of 2 [.mu.m], respectively; 4, a first clad layer of p-type Al.sub.0.25 Ga.sub.0.75 As crystal having a thickness of 2.5 [.mu.m], in the circular hole; 5, a light emitting layer of p-type Al.sub.0.05 Ga.sub.0.95 As crystal having a thickness of 0.3 [.mu.m]; 6, a second clad layer of n-type Al.sub.0.25 Ga.sub.0.75 As crystal having a thickness of 6 [.mu.m]; 7, an n-side electrode metal layer having a thickness of 0.2 [.mu.m] and being in oh/nic contact with the second clad layer; and 8, a circular window having a diameter of 90 [.mu.m] and provided by removing the ohmic contact layer 7 at a position opposing to the circular hole 3. Reference numeral 9 denotes a p-side electrode metal layer provided on the opposite surface of the substrate and being in ohmic contact with the substrate. Reference numeral 10 denotes a heat sink coupled to the p-side electrode metal layer 9.
When the p-side electrode metal layer 9 is grounded, and a negative potential is applied to the n-side electrode metal layer 7, a pn junction between the current blocking layer 2 and the first clad layer 4 is reverse-biased. For this reason, a current flowing through the light emitting layer 5 is substantially limited just above the circular hole 3. Since the band gap of the semiconductor crystal in the two clad layers 4 and 6 is larger than that in the light emitting layer 5, the injected minority carrier generated by the current injection into the light emitting layer is confined in the light emitting layer. Since the injected minority carrier is three-dimensionally limited to increase a minority carrier concentration, a current modulation response of the light output can be increased. In addition, a light emitting region is restricted to the light emitting layer. As described above, since the light having the limited diameter is emitted from the window 8, high optical coupling to a lens or an optical fiber can be easily obtained.
Another conventional light emitting diode is shown in FIG. 4. This light emitting diode has the same structure as that of the conventional light emitting diode shown in FIG. 3 in the three items (1), (2), and (3) described above. However, unlike the first prior art, in the light emitting diode shown in FIG. 4, a different crystal is used, and a so-called mesa-shaped structure in which a light emitting layer is left in only a region opposing to a window is used. That is, reference numeral 21 denotes a substrate of n-type InP crystal having a thickness of 100 [.mu.m]; 22, a buffer layer of n-type InP crystal having a thickness of 1.5 [.mu.m]; 23, a light emitting layer of p-type In.sub.0.47 Ga.sub.0.53 P crystal having a thickness of 0.8 [.mu.m]; and 24, a clad layer of p-type InP crystal having a thickness of 4 [.mu.m]. The substrate 21 and the buffer layer 22 act cooperatively as a first clad layer. A mesa portion 25 having a diameter of 30 [.mu.m] is formed extending from the surface of the clad layer 24 to the substrate by etching. The side surface of the mesa portion 25 is covered with an insulating film 26 of SiO.sub.2 having a thickness of 0.2 [.mu.m] except the top of the mesa structure that serves as a current path. Reference numeral 27 denotes a p-side electrode metal layer being in ohmic contact with the second clad layer 24; 28, an n-side electrode metal layer being in ohmic contact with the substrate 21; 29, a window provided in the n-side electrode metal layer 28 at a position opposite to the current path; and 30, a heat sink fixed on the p-side electrode metal layer 27.
In the light emitting diode having the structure of the first prior art, however, since the heterojunction for confining the minority carrier in the light emitting region is not present in the radial direction, the minority carrier may be laterally diffused from the periphery of the light emitting region. Therefore, the carrier concentration is relatively low in the light emitting region. As a result, since the light emitting region is surrounded by the residual region having a width of several .mu.m, which has the relatively low carrier concentration, that is, the long life time of the minority carrier, the response of the diode may be decreased. On the contrary, in the light emitting diode having the structure of the second prior art, since the dimension of the light emitting region in the radial direction is geometrically limited, the diffusion of minority carrier as described in the first prior art does not occur. Therefore, the response may not be decreased. However, since the side surface of the light emitting region including the pn junction is covered with the insulating film such as an oxide film with which the semiconductor crystals are not lattice-matched, the reliability of the diode is degraded.